1. Field of the Invention
The present invention generally relates to pulse code modulation terminal equipment, and in particular to a call processing which is to be executed in a pulse code modulation terminal equipment when a fault occurs in a pulse code modulation communication system.
Conventionally, a pulse code modulation (hereinafter simply referred to as a PCM) terminal equipment is used as a relay connected to an exchange in a communication system. As is well known, a PCM terminal equipment includes signal conversion panels (hereinafter referred to channel units), each of which includes an analog-to-digital (A/D) converter, a digital-to-analog (D/A) converter, and a signalling circuit. The signalling circuit processes various supervisory signals such as a seizure signal and an answer signal. Also, the PCM terminal equipment includes a multiplexer unit, which carries out time-division multiplexing and demultiplexing for voice signals and switch signals. For example, a PCM terminal equipment is constructed so as to conform to the CCITT (International Telegraph and Telephone Consultative Committe) recommendation G.732, which defines a 30-channel pulse code modulation system.
Further, the multiplexer unit ha the function of detecting a fault occurring in a multiplexed PCM signal supplied from another PCM terminal equipment. For example, when the multiplexer unit receives no multiplexed PCM signal from the PCM terminal equipment on the other side, it decides that a fault occurs in the system. Then, the signalling circuit in the source PCM terminal equipment sets a related line idle. Thereafter, the line in the idle state is blocked, and thereby prevented from being captured by an exchange.
It is noted that the multiplexer unit recognizes an interruption or hit of the received multiplexed PCM signal to be a fault thereof. It is to be noted that even when the multiplexed PCM signal is instantaneously interrupted, for example, for hundreds of milliseconds to a few seconds, this instantaneous interruption is recognized to be a fault, and the corresponding line is immediately set idle. In this case, if the related line is in an answer state, conversation is no longer continued. In the case where the line is set in the answer state, it is desired to keep the line i the connected state even when a hit occurs. Similarly, even if a hit occurs in a seizure state (off-hook state), the line is set idle. In this case, if the line is recovered from the fault, it is again switched to the connected state to establish a communication path. However, there is a possibility that the call may be connected to a different destination in error. From the above-mentioned reasons, in conventional PCM terminal equipments, it is difficult to use lines effectively and efficiently.